The present invention relates to data communication systems, and more particularly to devices and methods to eliminate the squelch-tail in radio receivers used in data communication systems.
When no carrier signal is being received in a radio receiver, the receiver output is noise. Since the noise serves no useful purpose, a squelch circuit is added to the receiver to mute the receiver audio when only noise is present. Muting is especially important in data receivers because the noise may be interpreted as data characters that can give false indications to users.
Squelch circuits operate by first recognizing the presence of noise, and then muting the audio output of the radio receiver in response thereto. Devices for sensing the noise, however, require a finite time period to recognize noise as such. During the interval between the end of the carrier signal and the start of muting, a burst of noise known as a squelch-tail is emitted by the radio receiver. The length of this tail is typically between 5 and 100 milliseconds.
The elimination of the squelch-tail and of the resulting erroneous data characters has been recognized as a problem, and proposals have been advanced for its resolution. In one such approach, activation of the squelch circuit is sped up to reduce the squelch-tail duration in strong signal conditions. (See, e.g., U.S. Pat. No. 4,359,780 to Day, dated Nov. 16, 1982.) Data Communication devices, however, do not distinguish actual data from noise incorrectly interpreted as data characters, and thus allow characters to be added to the data transmission. It is particularly important in data communication systems that each bit of transmitted data, even the last bit, be received and interpreted accurately.
In another approach, as seen in U.S. Pat. No. 4,153,878 to Osborn, dated May 8, 1979, data from the receiver is delayed by the period of time required to activate the muting in the squelch circuit. This approach, however, requires careful matching of the delay time to the response time of the squelch circuit. In addition, such delays may cause interference with subsequent transmissions.
Another approach uses an additional tone to indicate the period of signal transmission and to activate the squelch circuitry to eliminate the squelch-tail. (See U.S., e.g., Pat. No. 3,584,304 to Casterline, et al., dated June 8, 1971, and U.S. Pat. No. 3,654,555 to Ryan, et al., dated Apr. 4, 1972.) The use of a tone requires the addition of circuitry to add, detect, and then delete the tone. In addition, tone recognition, like noise recognition, takes time. In high speed data communication systems, such time delays may be unacceptable.
It is accordingly an object of the present invention to provide a novel system and method for eliminating the squelchtail in radio receivers used in data communication systems.
It is another object of the present invention to eliminate the squelch-tail in data communication system radio receivers without adding false data signals to, or deleting active data from, the end of the data transmission.
It is yet another object of the present invention to eliminate the squelch-tail and accurately interpret the last data bit in a data transmission.
It is a further object of the present invention to eliminate the squelch-tail in radio receivers used in data communication systems without including circuitry for a separate tone or delay line for squelch-tail elimination.
These and many other objects and advantages will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims and the following detailed description of preferred embodiments and read in conjunction with the appended drawings.